The present invention relates to a lens unit, and more particularly, to a lens unit suitable for use in a compact image capturing device. Further, the present invention relates to an image capturing device.
Most image capturing devices that are presently being used are digital cameras, which use charged coupled device (CCD) image sensors or complementary metal oxide semiconductor (CMOS) image sensors as image capturing elements. Thus, such an image capturing device must use a lens unit of which capacities differ from that of a so-called silver salt camera. For example, the sensitivity to diagonal incident light is lower in a CCD image sensor or CMOS image sensor than in a silver salt film. Thus, a CCD image sensor or CMOS image sensor must have a small chief ray angle (CRA), which is the incident angle of light rays entering an image capturing device. Due to the improvements made on the integration of image capturing elements, the image quality must be improved even for lens units used in compact camera or mobile phones. At the same time, such a lens unit must be reduced in size. Furthermore, a lens unit that is compact and used in a compact camera or mobile phone must have a structure that lowers manufacturing costs so that relatively inexpensive products can be supplied.
To improve the image quality, aberrations must be lowered. Thus, instead of a lens unit having a three-lens structure, which may easily be designed to be compact, it is desirable that a lens unit having a four-lens structure be used, which more easily suppresses aberrations. Japanese Laid-Open Patent Publication Nos. 2002-228922, 2006-309043, and 2007-11237 describe compact lens units having four-lens structures.
Japanese Laid-Open Patent Publication Nos. 2002-228922 describes a so-called front stop lens system in which a stop is located further toward an object side from the lens located closest to the image capturing subject (hereinafter referred to as the “object side”). In such a structure, it is generally known that when misalignment occurs in a lens located toward an image capturing element side (hereinafter referred to as the “image side”) from the stop, the misalignment greatly affects the image quality. As a result, there is a tendency for product defects to occur due to such misalignment. This lowers the manufacturing yield of the lens unit and increases manufacturing costs.
Japanese Laid-Open Patent Publication No. 2006-309043 describes a so-called rear stop lens system in which a stop is arranged further rearward from two lenses that are located at the object side. It is generally known that such a structure obtains a greater chief ray angle (CRA) than a front stop structure. It is strongly required that the CRA be small for a lens unit for a digital camera. Thus, the refractive index of each lens must be adjusted to decrease the CRA. However, when increasing the thickness of each lens or the gap between lenses to adjust the refractive index, the entire length of the lens unit is consequently increased. This enlarges the lens unit.
Japanese Laid-Open Patent Publication No. 2007-11237 describes a so-called middle stop lens system in which a stop is arranged between the first lens, which is located at the object side, and the second lens. This structure generally produces less defective products that are caused by misalignments than the lens system of Japanese Laid-Open Patent Publication No. 2002-228922 and has a smaller CRA than the lens system of Japanese Laid-Open Patent Publication No. 2006-309043.
Japanese Laid-Open Patent Publication No. 2007-11237 recites in paragraph [0013] that “by forming the two lenses at the object side from material having a relatively small linear expansion coefficient as expressed by conditional equations (3) and (4), a lens system may have small focusing movements and be applicable to temperature changes.” In other words, by forming the “two lenses at the object side” from a suitable material, the influence of temperature changes may be suppressed.
However, for compact cameras and mobile phones, it is required that every one of the lenses in lens units be plastic to lower weight and cost. The linear expansion coefficient of plastic is greater than glass. Thus, when the lenses are all plastic, the two lenses at the object side would not be formed from a material having a relatively low linear expansion coefficient.